1. Technical Field
The present disclosure relates to a three-dimensional integrated structure having a high shape factor and comprising a plurality of projecting elements obtained by a dry film photoresist.
The disclosure also relates to a method for forming such a three-dimensional integrated structure.
The disclosure refers, in particular, but not exclusively, to a three-dimensional integrated structure to be used in an electronic device, such as a microfluidic device, a microreactor or a sensor device, and comprising at least a substrate and a dry film photoresist, and the following description is made with reference to this field of application just for explanation convenience.
2. Description of the Related Art
As it is well known, dry film photoresists are laminated structures comprising different layers, and in particular a photosensitive material film, duly protected by a carrier film provided on its first or superior surface and a cover film provided on its second or inferior surface. In particular, the carrier film acts as a support for the photosensitive material film, while the cover film acts as a protection of the photosensitive material film until the dry film photoresist is applied on a substrate.
As an example, FIG. 1 shows a dry film photoresist realized according to the prior art and globally indicated with 101. In particular, the dry film photoresist 101 has a multi-layered structure, in particular comprising at least a photo-sensitive material layer or photoresist film 102 as a central layer; a cover film 103, bonded to a first surface of the photoresist film 102 and able to be released to laminate the dry film photoresist 101 on a substrate; and a carrier film 104 bonded to a second surface of the photoresist film 102 and able to be released after the photoexposure of the dry film photoresist 101.
The dry film photoresists are commonly used for optical photolithography, being usable for substrates of any size, nature and form according to a lamination process, similar to the one used for adhesive tapes.
They were introduced by DuPont in 1970 and mostly used for producing printed circuit boards (PCB), for manufacturing micro-mechanical systems (MEMS), for growing metals by electrochemical deposition, and also as permanent materials for bonding devices, in particular for realizing complex three-dimensional structures such as microfluidic devices or microreactors.
It should be remarked that, with respect to conventional liquid resists, dry film photoresists have the advantages of adhering to every type of substrates, which could have different shapes and sizes. These dry film photoresists have a very good thickness uniformity and allow a very high productivity which guarantees safety and low costs of production. In particular, the dry film photoresists can be processed using the multiple steps of roll to roll techniques, obtaining structures having thickness much greater than 100 μm and also comprising buried channels, structures that cannot be realized using liquid resists.
For example, the use of a dry film photoresist is described in the article entitled: “Fast Prototyping Using a Dry Film Photoresist: Microfabrication Of Soft-Lithography Masters for Microfluidic Structures”, published in the Journal of Micromechanics and Microengineering 17 (2007) N69-N74. According to this article, a dry film photoresist is used to manufacture soft-lithography masters used in microfluidic applications. In particular, it is indicated therein that the dry film photoresist is a convenient alternative to conventional microfabrication approaches based on liquid resists for fast-prototyping of microfluidic structures.
Dry film photoresists are classified according to two big categories:                aqueous processable dry film photoresists, which are developed in very much diluted solutions of water (H2O) and Sodium Carbonate (Na2CO3) or of water (H2O) and Potassium Carbonate (K2CO3);        solvent type dry film photoresists, which are acrylic monomers or polymers, namely comprising the acrylic group CH2═CH—C═O, being able to be developed in polar organic solutions.        
Moreover dry film photoresists can be permanent or non-permanent, in case they are left or not in a final electronic device, respectively.
In a well known manner, dry film photoresists are processed according a process comprising the steps of:                removing the cover film from the dry film photoresist;        laminating the dry film photoresist on a substrate, for instance using a pressurized hot roll able to guarantee a good adhesion of the exposed photoresist film to the substrate;        exposing the dry film photoresist for patterning it, for instance by using mask aligners or steppers;        releasing the carrier film from the dry film photoresist, uncovering the photoresist film; and        developing the photoresist film.        
As an example, FIGS. 2A-2F show the different steps of a method, according to the prior art, for forming a three-dimensional integrated structure 100 for an electronic device comprising a dry film photoresist which is duly patterned, in a negative form with respect to the final three-dimensional integrated structure to be obtained, such dry film photoresist being thus permanent and in particular of the solvent type.
In particular, FIG. 2A shows a first step wherein a dry film photoresist 101 is prepared by releasing its cover film 103 from the photoresist film 102, in particular in a manual or automatic manner.
After that, the method comprises a second step wherein the dry film photoresist 101, comprising the photoresist film 102 and the carrier film 104, is laminated on a substrate 105, in particular in correspondence of a flat surface 105A of the substrate 105 by using a pressurized hot roll 106, as shown in FIG. 2B.
The substrate 105 coupled to the photoresist film 102 is then covered by a mask 107 and subjected to a standard photoexposing step, leading to the patterning of the photoresist film 102, as shown in FIG. 2C. As already underlined, the photoresist film 102 is patterned in a negative form with respect to the final three-dimensional integrated structure to be obtained.
The method then comprises a step wherein the carrier film 104 is released from the photoresist film 102, as shown in FIG. 2D. The release of the carrier film 104 could be made manually or automatically.
Finally, the method comprises a photoresist development step, as shown in FIG. 2E, by using a developer 109. At the end of this step, the photoresist film 102 is removed from the substrate 105 according to its pattern, so as to realize a final three-dimensional integrated structure 100 comprising the substrate 105 and a plurality of elements that project from the flat surface 105A in an opposite direction with respect to the substrate 105, as shown in FIG. 2F. The projecting elements, globally indicated by 102A, are obtained by the duly patterned and developed photoresist film 102.
However, according to the different known methods for developing a photoresist film, a complete cleaning of the substrate 105 is usually not guaranteed.
In particular, several techniques are used to develop the photoresist film.
A first development technique, usually indicated as development in puddle, comprises the steps of:                spilling the developer one time or more from a programmed mobile nozzle on the substrate, being at rest or moving;        washing and drying the substrate.        
Also known is the so called spray development technique, which comprises the following steps of:                nebulizing the developer from a nozzle on the substrate, being at rest or moving;        providing a strong mechanical action for removing the photoresist film and residuals from the substrate surface.        
Both these first and second techniques involve a low consumption of the developer and allow to realize the development of the photoresist film as well as the washing and drying of the substrate in a same equipment. Nevertheless, they show several disadvantages, such as a non-uniformity of the distribution of the developer, possible random residuals of photoresist film on the substrate, as well as a low productivity due to the process on a single wafer. They also need specific equipment depending on the shape, size and thickness of the substrate.
Actually, the spray development technique is the most commonly used technique to develop roll photoresists having large thicknesses and it is very useful for defining geometrically complex structures, like coils, spirals and others, on high level topographies, in particular between ten and hundred microns.
Also known is a third development technique which comprises the steps of:                immerging the substrate inside a crystallizer, for a single wafer process, or inside a tank for wet etch, for a multiple wafer process; and        developing the photoresist film through chemical developers.        
The advantage of this technique is that it guarantees a very high productivity due to the possibility to process substrates having different shape and size, also multiple substrates simultaneously. Nevertheless, the photoresist film being processed according to this technique should have a limited thickness, since otherwise a very long development time would be required. Moreover, at the end of this development technique, the surface of the substrate turns out to be not very well cleaned, so that the lithographic resolution and the size uniformity of the final three-dimensional integrated structure so obtained are very limited.
An alternative embodiment of this type of development technique involves, during the developing step of the photoresist film through chemical developers, the use of ultrasounds generated, for instance, by an ultrasound source (in particular, at a frequency of 40 KHz).
This alternative embodiment has the advantages of allowing a high productivity while obtaining a dimensional uniformity as well as a more efficient removing of residuals from the substrate, due to the vibrations produced by the ultrasounds. Nevertheless, also this alternative embodiment has some drawbacks, mainly due to the more complex equipment to be used, which need an ultrasound generator and also software to provide the suitable programs, in addition to a conventional tank for the wet etch for ensuring a high productivity.
Moreover, acting mechanically on the substrate, this technique turns out to be not adequate for managing fragile, thin and flexible substrates.
The different known methods used for forming a three-dimensional integrated structure comprising projecting elements projecting from a substrate and obtained by a negatively patterned and developed type solvent dry film photoresist have some drawbacks, mainly tied to:                the difficulty in obtaining shape factors, which is defined as the ratio of the cross section area of the projecting elements to their height, greater than 6; in particular, the shape factors are at present quite far form the shape factors being obtained by using liquid photoresists which are greater than 10;        the difficulty in obtaining profile of the walls of the projecting elements obtained by the dry film photoresist that could be modified as desired;        the difficulty in having low cost production.        
As it is clear from the above explanation, the most sensitive step of the known methods is tied to the techniques for the development of the photoresist film.